1. Field of Art
This invention relates to apparatus used in the drilling of earthen boreholes, commonly called “wells,” from floating drilling vessels. With further specificity, this invention relates to apparatus used in connection with the deployment of subsea wellheads, the protection of critical sealing areas thereon, and the placement and cementing of the associated casing string.
2. Related Art
Onshore or in shallow offshore waters, wellhead equipment is situated so as to be fairly close to (although nearly always somewhat below) the level of the rig floor. In particular, on offshore wells in water shallow enough that a bottom-supported, jackup type drilling rig can be used, a length of conductor casing extends upwardly from the sea floor, and helps support the wellhead which is generally mounted on the surface casing string, above the surface of the water a desired distance. The production “tree” (a multiple valve assembly) is therefore situated above the water level. A similar wellhead arrangement is used when wells are drilled by a platform drilling rig from a drilling/production platform.
In deeper waters, however, floating drilling rigs (either semi-submersible rigs or drillships) must be used. Here, the wellhead is situated on or near the sea floor and is therefore often referred to as a “subsea wellhead.” For the illustrative purposes of this description, it will be assumed that the subsea wellhead is connected to the surface casing string. The blowout preventer (“BOP”) stack clamps around, and effects a pressure and fluid seal with, the wellhead. Typically, a grooved profile in the bore of the BOP stack engages a similarly shaped, mating profile on the exterior of the subsea wellhead. Pressure integrity is typically achieved by a metal-to-metal seal between a seal area in the subsea wellhead, usually a beveled area around the inner circumference and near the top edge of the subsea wellhead (often referred to in the industry as the “AX bevel”), and a similar profile within the BOP stack bore. It can be appreciated that maintenance of the pressure seal between the BOP stack and the wellhead is of crucial importance to maintaining control of the well. In addition, if and when the subsea well is completed, a subsurface tree clamps to the wellhead and (like the BOP stack) must effect a pressure seal therebetween.
Critical to creating and maintaining this pressure seal is the beveled seal area on or near the upper edge of the subsea wellhead. This is generally a highly polished profile that must be protected during handling of the wellhead, lest it be scarred or otherwise damaged. If damaged, repairs can be very expensive, in addition to the rig downtime which must be incurred for the repair. In short, there are tremendous economic incentives to protect the subsea wellhead seal area.
However, the usual operations associated with running the subsea wellhead and surface casing string, and cementing the surface casing string, create numerous opportunities for damage to the subsea wellhead seal area. For illustration only, a typical sequence of operations will now be described.
One of the first steps in drilling a subsea well is running of “conductor pipe” (which is usually either drilled or jetted into place, and is typically 30″ in diameter), and landing same with a subsea guide assembly in place on the sea floor. The surface hole (typically 26″ in diameter) is then drilled to the setting depth for the surface casing string (which for illustration may be considered as 20″ casing), typically to a depth of about 2000″ below the sea floor. The 20″ surface casing string is then run down through the 30″ conductor (the joints of the 20″ casing string joined by either threaded or welded connections) until the last joint (save for the partial or full joint usually made up to the subsea wellhead beforehand, which will be referred to as the “landing joint”) is disposed in the casing “spider,” which is resting on the rotary table of the rig.
The next series of operations involves picking up the subsea wellhead along with its connected landing joint, and connecting that combined wellhead and landing joint to the remainder of the surface casing string. Then, the subsea wellhead must be lowered to a position substantially at the level of the rig floor, and supported there, while a length of drill pipe—the “cementing string”—is run inside the surface casing string to near its lower end. The subsea wellhead running tool (typically a cam actuated tool which engages a grooved profile in the subsea wellhead bore) is then latched into the subsea wellhead, and the entire assembly—that is, the subsea wellhead, the surface casing string, and cementing string—is lowered into the borehole on the drill pipe and the subsea wellhead is landed in the guide assembly on the sea floor. The surface casing is then cemented in place.
This description of operations identifies the need for an apparatus adapted to be disposed on the rig floor, to suspend the subsea wellhead and surface casing string therefrom, and which additionally provides a means for supporting drill pipe slips (and the weight of the cementing string) in order to run the cementing string inside of the surface casing. Prior art apparatus which sought to address these needs comprised a large two-piece circular clamp, each of the two pieces forming a half-circle. The two half-circles were placed around the subsea wellhead, then bolted together, typically with three nut and bolt assemblies per side. A grooved profile in the clamp bore engaged the grooved profile on the exterior of the subsea wellhead. The clamp could then be set upon the rig floor, thereby suspending the subsea wellhead and surface casing. Next, a circular plate having a diameter sufficient to span the diameter of the clamp bore was placed atop the clamp. This plate had a U-shaped cutout from its center to an edge, to permit it to be removed from around the drill pipe; however, no provision was made to prevent the plate from moving side to side on the clamp. A drill pipe slip bowl was then placed atop the plate, and the cementing string was then run in conventional manner with the slips.
This prior art apparatus presented several problems and drawbacks. First, bolting and unbolting the two halves of the clamp was time consuming and posed an increased risk of injury to rig workers. Second, the drill pipe slip bowl was not secured to prevent side-to-side movement within the subsea wellhead, since it was simply resting atop the base plate; therefore, particularly with the rocking movement often present on a floating drilling rig, the drill pipe cementing string was subject to striking the inner wall of the subsea wellhead bore, and more importantly to striking and damaging the bevel seal area of the subsea wellhead. The attendant potentially tremendous direct and indirect costs resulting therefrom are readily appreciated.
What is needed, therefore, is an apparatus which is adapted to quickly and safely engage the subsea wellhead to suspend the wellhead and the surface casing string from the rig floor, and thereafter to quickly and safely disengage from the subsea wellhead. Further, the apparatus should comprise a drill pipe slip bowl assembly which is held securely centered within the subsea wellhead bore, thereby centering the cementing string within the subsea wellhead bore, and minimizing (if not eliminating) the possibility of the cementing string damaging the seal area of the subsea wellhead.